Resonant Raman scattering in polyacetylene under hydrostatic pressure

The results of resonant Raman scattering experiments on trans-poly-acetylene under hydrostatic pressure are reported. The measurements were performed in a diamond anvil cell. The spectra could be measured up to 44 kbar. The pressure dependence of the 1295 cm^-1 line was measured in a sapphire cell up to 17 kbar. The results show that the changes in the phonon frequencies are very small. By comparing the pressure dependence of the Raman bands with their dependence on the photon energy of the exciting laser line it is possible to determine the pressure variation of the electronic energy gap. The results are consistent with previous measurements of the absorption spectrum under hydrostatic pressure which were carried out up to 13.5 kbar. The gap is found to decrease rapidly with pressure but the decrease tends to saturate at high pressures. The results are consistent with a model in which chain-chain interaction plays a dominant role.     

Finite-size effects in tunneling between parallel quantum wires

We present theoretical calculations and experimental measurements which reveal finite-size effects in the tunneling between two parallel quantum wires, fabricated at the cleaved edge of a GaAs/AlGaAs bilayer heterostructure. Observed oscillations in the differential conductance, as a function of bias voltage and applied magnetic field, provide direct information on the shape of the confining potential. Superimposed modulations indicate the existence of two distinct excitation velocities, as expected from spin-charge separation.     

The structuredness of solvents

The parameterization of the structuredness of a solvent as the difference between its molar heat of vaporization, Δ_vap H ^° and the sum of its internal donor-acceptor interactions (in terms of DN and AN) and its vdW interactions (in terms of Δ_vap H ^° of n-alkanes of the same molar volume) is criticized. As an alternative, the excess of the reduced Trouton constant, Δ_vapS^°/R over 12 and of Kirkwood's dipole correlation parameter g over 1.7 are suggested as criteria for the decision of whether a solvent is structured, both having to be met. Conversely, if either Δ_vapS^°/R is less than 11.6 or g is less than 1.3, then the solvent is to be considered unstructured. Exceptions are discussed.     

Preferential Solvation of Ions in Mixed Solvents. 5. The Alkali Metal, Silver, and Thallium(I) Cations in Aqueous Organic Solvents According to the Inverse Kirkwood-Buff Integral (IKBI) Approach

The inverse Kirkwood-Buff integral (IKBI) approach is applied, as far as the relevant data exist in the literature, to the preferential solvation of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Ag⁺, and Tl⁺ in aqueous mixtures of methanol, ethanol, 1,2-ethanediol (EG), acetonitrile, formamide, N,N,-dimethylformamide (DMF), N,N,N′,N′,N″,N″-hexamethyl phosphoric triamide (HMPT), and dimethyl sulfoxide (DMSO). In aqueous EG and formamide the preferential solvation is very small. Water is the preferred solvent in the solvation shells in aqueous methanol, ethanol and acetonitrile (except for Ag⁺ in the latter solvent) but the co-solvent is preferred in aqueous mixtures of DMF, HMPT, and DMSO over most or all of the composition range. For the latter three mixtures the larger donicity of the co-solvents causes their preference, whereas where water is preferred over other protic solvents, it is the small size of the water molecules that appears to be the cause.     

The structuredness of solvents. 2. Data for ambient conditions

In view of the inadequacy of Trouton's constant for the expression of the structuredness of solvents at ambient temperatures, alternative measures have been sought. Calculations of the entropy deficit _vapS⁰/R of solvents at ambient conditions relative to their vapors and compared to presumably unstructured alkanes with the same number and arrangement of skeletal atoms have been made. Data for nonpolar, dipolar aprotic, non-hydroxylic protic and hydroxylic solvents are presented, with _vapS⁰/R>2 representing structured solvents. Empirically, the heat capacity density of solvents at ambient temperatures, [C _p (l)-C _p (g)]/V], is another meaningful measure of the structuredness of solvents, compatible with the former one. Values>0.6 J-K^–1-cm^–3 signify a structured solvent.     

Preferential Solvation in Mixed Solvents

The Kirkwood–Buff integrals and the volume-corrected preferential solvation parameters for the first solvation shell of binary mixtures of tetrahydrofuran with many organic solvents, calculated from reported thermodynamic data at the temperatures for which these data were available, are reported. The co-solvents include c-hexane, methyl-c-hexane, n-heptane, i-octane, benzene, toluene, ethylbenzene, 1-chlorobutane, dichloromethane, 1,2-dichloroethane, chloroform, 1,1,1-trichloroethane, tetrachlorom-ethane, tetrachloroethene, hexafluoro benzene, ethanol, 1-propanol, 2-propanol, dibutyl ether, acetic acid, acetone, dimethyl sulfoxide, tetramethylene sulfone (sulfolane), acetonitrile, pyrrolidine, and triethylamine. The preferential solvation parameters of these mixtures are discussed in terms of the interactions that occur.     

The solubility and solvation of salts in mixed nonaqueous solvents. 1. Potassium halides in mixed aprotic solvents

The solubilities of potassium fluoride, chloride, and bromide in acetonitrile, N,N-dimethylformamide, and dimethylsulfoxide and in binary mixtures of these solvents were determined at 25°C. The standard molar Gibbs free energies of solution, _solnG°, in the neat solvents were related to the polarizabilities and basicities of the anions and the dipole moments and acidities of the solvents. The values of _solnG° in the mixtures were fitted by expressions from the quasi-lattice quasi-chemical theory. The mean number of each kind of solvent in the nearest environment of the ions was obtained from these results.     

On the Molar Volumes and Viscosities of Electrolytes

The b _V coefficient of the term linear with the concentration of the apparent molar volume ^ϕ V of electrolytes in water and several non-aqueous solvents is examined. Its relationship with the B _η coefficient of the corresponding term in the relative viscosity of these electrolyte solutions is explored. Positive correlations are found in some cases as expected, but in others, where crowding of the solvation shells occurs on increasing concentration, such correlations fail.